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Uncovering the universe’s invisible secrets

Like many people, I was afraid of the dark as a child. I would struggle to sleep in a pitch-black room, my imagination projecting anything scary onto the space I couldn’t see. Perhaps there are evolutionary reasons for this common fear to do with nocturnal predators, but there’s also a very simple one: sight is the primary sense most of us rely on to collect information about our surroundings. If we can’t see what’s around us, then how can we be sure there are no monsters lurking in the dark?

In astronomy, it turns out, invisible monsters are very real – even if they aren’t the kind that most children are scared of. From black holes to the ancestors of dead galaxies, these elusive cosmic characters are the focus of The Invisible Universe: Why There’s More to Reality than Meets the Eye, by Matthew Bothwell, an astronomer and science communicator at the University of Cambridge, UK.

In the introduction, Bothwell makes a compelling case for studying the unseeable by use of a striking analogy. If we consider the spectrum of visible light to be a single octave on a piano, with red light being the note middle C and blue light, roughly half the wavelength, being the C an octave above, then how far does the full spectrum of electromagnetic (EM) radiation extend? The answer is 65 octaves, “as much as nine grand pianos standing in a line”.

It’s a humbling fact for a species that uses sight as its main sense – and it doesn’t stop there. There are astronomical spectacles that would still elude us even if we could see across all 65 octaves of light. After all, we can’t see black holes because no light of any wavelength can escape them, while dark matter doesn’t seem to interact with EM radiation at all. Meanwhile, gravitational waves are ripples in the fabric of space-time itself rather than anything to do with EM radiation, and all we know about dark energy is that it is causing the universe to expand at an accelerating rate. The Invisible Universe has chapters dedicated to all of these phenomena, describing them vividly while explaining the physics behind them in an accessible manner.

Bothwell approaches each chapter like a detective story: he introduces some unexpected observation, follows the development of theories to explain it, and relates their triumphs or failures. One chapter that particularly swept me along is “Monsters in the dark: the quest to find the Universe’s hidden galaxies”.

Bothwell explains that there is a class of galaxies in the universe that are gargantuan even by galaxy standards – one such behemoth is “big enough to swallow the Milky Way, Andromeda, and all the space between”. But strangely, all the galaxies this size seem to be dead – they no longer actively form stars. The rest of this chapter reads like a cosmic murder mystery, as we look back in time – by looking deeper into space – in search of these galaxies’ ancestors and what killed them off.

Puzzlingly, there are no candidates visible in the images of deep space beamed back by the Hubble Space Telescope that are nearly extreme enough to be those predecessors. But non-visible sub-millimetre observations reveal a new type of galaxy in the early universe, shrouded in dust, that produced stars ten times more rapidly than even “starburst” galaxies nearby. These ancient star factories are excellent candidates for the dead galaxies’ ancestors, but how did they all die off? It is a testament to how much I enjoyed this chapter that I feel I shouldn’t give this spoiler away – but I will say that the resolution does not disappoint.

I also enjoyed the stories of the scientists and engineers that Bothwell weaves throughout the book. One I hadn’t heard before concerned Karl Jansky and Grote Reber, who were early pioneers of radio astronomy. In the early 1930s, Jansky was an engineer working at Bell Labs in the US, trying to get rid of the problem of radio static. While studying this noise, he discovered a type of signal that repeated every 23 hours and 56 minutes.

Bothwell explains that this is a tell-tale sign of something from beyond the solar system. Although we think of a day as being 24 hours long, it actually only takes 23 hours and 56 minutes for the Earth to rotate once relative to the Milky Way. During that time the Earth has moved a bit further around the Sun, so the planet needs to rotate for another four minutes for the Sun to reach the original point in the sky. But in 23 hours 56 minutes, we’re lined up with our galaxy again. Therefore a signal with this period is likely to be coming from outside of the solar system – indeed, Jansky was detecting radio waves from centre of the Milky Way.

Incredibly, the astronomy community as a whole took little notice of Jansky’s results when he first published them. Bothwell describes this as being for one simple reason: “the world of radio engineering was just too far removed from the world of astronomy”.

But Reber – an engineer who designed electric equipment for radio broadcasts – was fascinated with this noise from outer space. He built his own radio telescope in his back garden in Chicago, and, from the mid-1930s to the mid-1940s, was “the only radio astronomer in the world”.

Reber took courses in physics and astronomy at his local university, to help him understand his observations, and he was the first person to detect radio waves emanating from hot gas clouds where stars were being born. According to Bothwell, Reber was “the last of the amateur ‘outsider’ scientists”, who “through painstaking and meticulous work managed to change the scientific world”.

This story and many others throughout the book underline the crucial importance of technology for progress in astronomy – and the current technology is remarkable. For example, to detect gravitational waves, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has to be able to sense a squeeze and stretch of less than the diameter of a proton, and to image black holes’ event horizons, the Event Horizon Telescope achieves a resolution equivalent to reading a newspaper in New York using a telescope in London.

So the feats of engineering that humans have achieved to detect astronomical phenomena are perhaps as awe-inspiring as the phenomena themselves. And Bothwell reminds us that there’s more to come, with projects underway to take capabilities to the next level across all fields of astronomy. For example, the European Space Agency is currently building the first space-based gravitational wave detector. The Laser Interferometer Space Antenna (LISA) will be able to detect gravitational waves with much longer wavelengths than LIGO can because it’s mirrors will be an incredible 2.5 million kilometres apart compared to LIGO’s 4 km.

By highlighting the future of astronomy, the book pre-empts its own expiry date, well aware that it is likely an incomplete tour of the invisible, with future technology destined to uncover yet more monsters in the dark. When the book was written, the James Webb Space Telescope had not yet been launched. Now it is in space, already sending back breath-taking infrared images of the universe.

But I think the book’s timing is actually perfect. In an era when so many pioneering new projects are underway, it has reignited my excitement about what weirder and more wonderful curiosities they might find. And, even if it means the book becomes slightly out-of-date, I get the strong impression that the author is pretty excited about that too.

  • 2021 Oneworld Publications 320pp £18.99hb
  • 2022 Oneworld Publications 320pp £10.99pb

Aftermath of explosive stellar merger seen in a new light

The aftermath of the merger of a neutron star with another star has been observed using millimetre-wavelength light for the first time. The distant merger occurred when the universe was about 5.5 billion years old and was immediately followed by one of the most energetic short-duration gamma-ray bursts (SGRBs) ever spotted by astronomers. It also left behind one of the most luminous afterglows ever seen. These latest millimetre-wavelength observations of that afterglow could help astronomers understand how heavy elements are forged in such cataclysmic mergers.

The SGRB is called GRB 211106A and its gamma-rays were spotted in 2021. Now, Tanmoy Laskar and colleagues have used the Atacama Large Millimetre/submillimetre Array (ALMA) radio telescope in Chile to observe millimetre-wavelength light from the afterglow. In the electromagnetic spectrum, this light falls between the infrared and microwaves.

Explosive mergers involving neutron stars are believed to forge heavy elements like platinum and gold. So, understanding how these mergers proceed is important to understanding how galaxies evolve – and ultimately how heavy elements end up in planets like Earth.

“Very few SGRB afterglows have been detected at radio wavelengths. This is because, although they are very luminous, these explosions take place in distant galaxies, which means the light from them can be quite faint for our telescopes on Earth,” explains Laskar, who will soon join the University of Utah. “Only about half a dozen SGRB radio afterglows are known. And despite almost two decades of searching, none had been detected at millimetre wavelengths.”

Useful emissions

Laskar explains that finding millimetre emissions from an SGRB is particularly useful because the light is unaffected by passing through ionized gas in the Milky Way, something that can make the interpretation of observations of longer wavelength radio waves challenging. Millimetre-light is also immune to the quantum effects that can make the interpretation of high-energy X-rays from distant sources difficult.

Team member Wen-fai Fong, who is at Northwestern University, adds that millimetre wavelengths allow astronomers to “see through” obstructing material that is normally opaque to other wavelengths. “These observations revealed a large amount of dust in the vicinity of this gamma-ray burst,” she adds. “This explains why we did not observe any visible light from the burst.”

Indeed, the combining of observations in varying wavelengths of light was the key to revealing a clearer picture of this powerful event. Northwestern’s Genevieve Schroeder tells Physics World that combining millimetre observations of GRB 211106A with X-ray data showed the team just how energetic and wide the gamma-ray burst was.

Laskar adds, “Learning about these properties helps us better understand the progenitors of these extreme explosions — neutron star mergers”.

Fast moving jets

“When the stars merge, the resulting explosions are accompanied by jets of material moving at close to the speed of light,” says Laskar. “When one of these jets is pointed at Earth, we observe a short pulse of gamma-ray radiation, an SGRB.”

The gamma-ray signal is fleeting – lasting just a fraction of a second – so it is difficult to use a SGRB alone to pinpoint the location of the merger. Fortunately, when the jet strikes gas surrounding the merger, it creates a longer-lasting afterglow that can be seen by astronomers. “Capturing the afterglow light is essential for figuring out which galaxy the burst came from and for learning more about the burst itself,” Laskar explains.

Nevertheless, Schroeder says that the team’s success was not guaranteed. “This observation was the first time we have pointed ALMA at a SGRB, and we were only able to detect the afterglow due to ALMA’s remarkable sensitivity. Previous millimetre observations of SGRBs have resulted in non-detections due to the less sensitive telescopes, so this burst really highlights ALMA’s amazing capabilities.”

Because GRB 211106A has been studied across multiple wavelengths as the afterglow faded, Fong says that the team probably will not look at this particular merger with ALMA again.

While gravitational waves have been seen from neutron star mergers, they were not seen from GRB 211106A. This is because the signal would have been too faint for existing gravitational-wave detectors to observe. However, Fong points out that future generations of gravitational wave detectors will soon be able to detect mergers as distant as GRB 211106A.

“That will be a really exciting era, as it will become routine to detect SGRBs in tandem with their gravitational waves.”

The team’s findings are described in a paper on arXiv. The paper has been accepted for publication in The Astrophysical Journal Letters.

Octopus-inspired glove grabs underwater objects using LIDAR

Inspired by the way the skin on octopus arms works, researchers at Virginia Tech in the US have developed a new rapidly switchable adhesive that sticks securely to objects underwater. The material could find use in robotics, healthcare and in manufacturing for assembling and manipulating wet objects.

Adhesives that work underwater are difficult to make. This is because the hydrogen bonds and van der Waals and electrostatic forces that mediate adhesion in dry environments are much less effective in water. The animal world, however, contains lots of examples of strong adhesion in moist conditions: mussels secrete special adhesive proteins, creating a sticky plaque to attach to wet surfaces; frogs channel fluid through structured toe pads to activate capillary and hydrodynamic forces; and cephalopods like the octopus use suckers to adhere to surfaces via suction.

Strong adhesive bond

Cephalopod grippers are particularly good at holding things underwater. Octopi, for example, have eight long arms covered with suckers that can grab onto objects like prey. Shaped like the end of a plumber’s plunger, the suckers adhere to an object, quickly creating a strong adhesive bond that is difficult to break. “The adhesion can be quickly activated and released,” explains study team leader Michael Bartlett, “and the octopus controls over 2000 suckers across eight arms by processing information from diverse chemical and mechanical sensors.”

Indeed, an octopus’ sensing apparatus consists of a photoreception system that uses its eyes; mechanoreceptors that detect fluid flow, pressure, and contact; and chemoreception tactile sensors. Each sucker is independently controlled to activate or release adhesion – something that does not exist in synthetic adhesives.

The new Virginia Tech octopus-inspired adhesive consists of a silicone elastomer stalk capped with a stretchable pneumatically-actuated elastomer membrane to control adhesion. The stalk is made by 3D printing moulds and the silicone elastomer is then cast and cured. The adhesive element is connected to a pressure source that supplies positive, neutral, and negative pressure to control the shape of the active membrane.

“This design allows us to switch adhesion 450 times from the on to off state in less than 50 ms,” says Bartlett. “We tightly integrated these adhesive elements with an array of micro-LIDAR optical proximity sensors that sense how close an object is.”

The researchers then connected the suckers and LIDAR through a microcontroller for real-time object detection and adhesion control.

Glove with synthetic suckers and sensors

Underwater, an octopus winds its arms around objects and can attach to a variety of surfaces, including rocks, smooth shells and rough barnacles using its suckers. Bartlett and colleagues mimicked this by making a glove with synthetic suckers and sensors tightly integrated together. This device, dubbed Octa-glove, can detect differently-shaped objects underwater. This automatically triggers the adhesive so that the object can be manipulated.

“By merging soft, responsive adhesive materials with embedded electronics, we can grasp objects without having to squeeze,” said Bartlett. “It makes handling wet or underwater objects much easier and more natural. The electronics can activate and release adhesion quickly. Just move your hand toward an object, and the glove does the work to grasp. It can all be done without the user pressing a single button.”

These capabilities, which mimic the advanced manipulation, sensing and control of cephalopods, could find applications in the field of soft robotics for underwater gripping, applications in user-assisted technologies and healthcare, and in manufacturing for assembling and manipulating wet objects, he tells Physics World.

Several gripping modes

In their experiments, the researchers tested several gripping modes. They used a single sensor to manipulate delicate, lightweight objects and found that they could quickly pick up and release flat objects, metal toys, cylinders, a spoon and an ultrasoft hydrogel ball. By then reconfiguring the sensors to that multiple sensors were activated, they could grip larger objects such as plate, a box and a bowl.

The Virginia Tech team, reporting its work in Science Advances, says that there is still much to learn, both about how the octopus controls adhesion and manipulates underwater objects. “If we can better understand the natural system, this will allow to create more advanced bio-inspired, engineered systems,” says Bartlett.

Babies have an innate understanding of symmetry, we fall foul of chorizogate, astronomers love science fiction

You are never too young to start learning about physics – and now researchers in Europe have backed this up by showing that babies just seven months old have a grasp of symmetry.

Irene de la Cruz-Pavía at the University of the Basque Country, Judit Gervain of the University of Padua and colleagues showed nearly 100 babies mosaic patterns in a study that was done at the University of Paris. The linear patterns had varying degrees of symmetry (see figure) and the babies were able to discriminate between patterns that were structurally symmetric and patterns that were asymmetric.

“Babies as young as seven months have a robust, automatic ability to detect that a structure is symmetrical. This ability coincides with those found in studies we conducted using other stimuli, such as sign language or speech, demonstrating that babies are simply very good at detecting structures and regularities,” says de la Cruz-Pavía.

I believe it was the Nobel laureate Philip Anderson who famously said, “It is only slightly overstating the case to say that physics is the study of symmetry,” so it looks like babies are physicists by nature.

The research is described in a paper in PLOS ONE.

Spicy space sausage

Like many other media outlets, Physics World gleefully reported last week how a photograph of a slice of chorizo had gone viral after prominent French physicist Etienne Klein joked on Twitter that it was the latest image from the James Webb Space Telescope.

Klein apologized for the prank after admitting it was just a piece of sausage from his fridge, but it looks like we need to say sorry too. That’s because, if we’d bothered to dig a tiny bit deeper, we’d have realised that the image had originally been tweeted a day before Klein by theoretical astrophysicist Peter Coles at Maynooth University in Ireland.

As Coles explains on his Telescoper blog, he posted the image on 30 July with the caption “Those JWST images just get better and better”, whereas Klein tweeted the same photo on 31 July. Coles, however, admits that it wasn’t even his image in the first place. “I didn’t make the picture and don’t remember where I got it from, though it was probably here,” he writes on his blog, referring to a tweet from 2018 by a user called Jan Castelmiller, who claimed the chorizo was the red-coloured “blood” Moon seen during a lunar eclipse.

The mystery of the stellar sausage deepens.

Sci-fi confession

I have a confession to make about science fiction: I can take it or leave it. Although I did a PhD in physics and I have since been writing about science for decades, I don’t really find it that compelling in an artistic or literary sense. Sure, I loved watching the original Star Trek series when I was a kid and I do enjoy the occasional sci-fi blockbuster. But you could probably count the number of sci-fi novels I have read over the past 25 years on one hand.

As a result, I had assumed that most people who dealt with science on a professional level shared my sci-fi ambivalence. So, I was surprised to discover that there appears to be a strong correlation between having a love for science fiction and being a professional astronomer. That is the finding of Elizabeth Stanway, who is an astronomer at the UK’s University of Warwick. Stanway conducted two surveys that asked astronomers about their attitudes towards science fiction and whether an interest in science fiction influenced their decision to pursue careers in science.

In one survey of more than 200 UK astronomers, a whopping 94% of respondents expressed an interest in science fiction. Furthermore, 69% said that science fiction had influenced their life or career choices. Writing in a paper on arXiv that reports her findings, Stanway says “This study provides strong statistical evidence for the role of science fiction in influencing the adoption of astronomical careers”.

Ask me anything: Taghi Amirani – a physicist turned documentary-film maker

What skills do you use every day in your job?

Physics and film-making have been intertwined throughout my career since I was an undergraduate. In 1984 I made the unusual suggestion to the physics department at the University of Nottingham that my final-year project should be a film instead of a lab project. Rather than conduct a physics experiment that needed writing up, I would make a documentary about black holes.

After weeks of back-and-forth and negotiations that went all the way up to the heads of the science faculty, the Nottingham physicists displayed remarkable open-mindedness and agreed. Shades of Black became my first film and launched 38 years of film-making adventures. Every skill I developed in making that project a reality, I use to this day.

Taking a leap of faith into the unknown, honing persuasive powers to attract funders and collaborators, hard-nosed and persistent research, perseverance in the face of obstacles and trusting my instincts are some of the skills that have helped me. All of these culminated in my latest film, a theatrical feature called Coup 53. Every scientific endeavour requires an analytical mind driven by intense curiosity. So does documentary-making. Shades of Black was about going back in time to uncover the secrets of the universe. Coup 53, the story of the 1953 MI6/CIA coup in Iran, is about uncovering dark secrets buried deep in history. 

What do you like the best and least about your job?

What I like best about being a documentary maker is that I can pick a subject I am curious about and take a deep dive into it until I find a compelling story worth telling. The serendipity of unexpected connections and revelations is pure joy when combined with artistic expression to tell a story that is personally meaningful and resonates with a wide audience. You take what seems like a personal obsession, turn it into a film that requires an army of collaborators to bring it to fruition, and then stand at the back of a cinema and watch it grip and move total strangers gathered in the dark. That human connection is precious beyond words. 

For the duration of a production, you become so immersed and absorbed in a new subject that you can almost pass as an expert, albeit in a small sub-section. The learning never stops. This was particularly true in making Coup 53, for which I was privileged to work with the legendary Walter Murch (Apocalypse Now, The Godfather, The English Patient). It was physics that brought us together. Walter was editing Particle Fever – a documentary about the discovery of the Higgs boson, when we met in New York in 2012. Bonding over physics and film-making, I never imagined that I would end up working with Walter on the most important film of my life. He has not only made me a better filmmaker, but our collaboration made me a better human being. This is the kind of experience that transforms a job into a calling.

What I like least about my job is that I love it so much that I occasionally work for no pay. This is a habit I am working hard to break. 

What do you know today that you wish you knew when you were starting out in your career?

Uncertainty is good. Having a five-year plan or a 10-year plan is pointless if you’re a creative soul. Don’t worry about having a career. Find what you love and keep on doing it. Trust your process and go with the flow because things will almost always work out. Embrace your mistakes. Take more risks. Sometimes not getting what you want is the best thing that can happen to you. 

Physicists identify most complex protein knots

Scientists in Germany and the US have predicted the most topologically complex knot ever found in a protein using AlphaFold, the artificial intelligence (AI) system developed by Google’s DeepMind. Their complete analysis of the data produced by AlphaFold also revealed the first composite knots in proteins: topological structures containing two separate knots on the same string. If the discovered protein knots can be recreated experimentally it will serve to verify the accuracy of predictions made by AlphaFold.

Proteins can fold to form complex topological structures. The most intriguing of these are protein knots – shapes that would not disentangle if the protein were pulled from both ends. Peter Virnau, a theoretical physicist at Johannes Gutenberg University Mainz, tells Physics World that there are currently around 20 to 30 known knotted proteins. These structures, Virnau explains, raise interesting questions around how they fold and why they exist.

A protein’s shape can be closely linked with its function, but while there are a few theories on the functionality and purpose of protein knots there is little hard evidence to back these up. Virnau says that they might help to keep the proteins stable, by being particularly resistant to thermal fluctuations, for instance, but these are open questions. While protein knots are rare, they also appear to be highly preserved by evolution.

“If a knotted protein exists, for example, in yeast, there is a high likelihood that it is also knotted in the corresponding protein in humans,” Virnau explains. “So, these are structures that have been around for hundreds of millions of years.”

A long-standing problem in protein knot research has been finding and identifying protein knots. While complex protein structures have been experimentally determined in the laboratory, this can be challenging and time consuming. Recently, DeepMind developed an AI system known as AlphaFold that it claims can predict protein structures with incredible speed and precision. The deep-learning system works on a large database of known proteins and their amino acid sequences. It uses those sequences and information on the primary structure of amino acids to predict the three-dimensional structures of the proteins. Its training is based around evolutionary, physical and geometric constraints of protein structures.

AlphaFold has predicted several hundred thousand protein structures, most of which have not yet been catalogued. In this latest work, published in Protein Science, Virnau and his colleagues searched AlphaFold’s databank for previously unknown complex protein knots. They discovered nine new knots. This included the first 71-knot – a knot with seven crossing points that is the most topologically complex knot ever found in a protein.

The researchers also found several six-crossing composite knots. These each contain two trefoil knots, which are knots with three crossings. They also discovered two previously unknown knots with five essential crossings, a 51-knot and a 52-knot.

The team is now working with biochemist Todd Yeates, at the University of California Los Angeles, to create the proteins identified by AlphaFold experimentally to confirm that they form the predicted topological structures. “I’m quite confident that we will be able to confirm these structures experimentally,” says Virnau.

If these topologically challenging structures can be created experimentally it would show that AlphaFold is working as expected and provide confidence in its predictions of less complex protein shapes. “The protein knots might only be a minor aspect of this, but it may nevertheless serve as a validation of these tools in general,” Virnau explains.

In the future it might be possible to use these AI tools for protein engineering. Proteins could be designed containing knots and other complex structures that provide them with functionality for specific tasks, although this is at least a few years away.

Building a science centre that will inspire local communities, musing over first lines in science books

Many physicists are keen to share their enthusiasm for science with the public, and this often involves participating in events at different venues across a community. In this episode of the Physics World Weekly podcast, we meet four people in the Canadian city of Guelph who believe that their community’s appetite for science is so great that it warrants a dedicated science centre.

All four are all members of Royal City Science, which was founded in 2020 with the aim of building a science centre in Guelph. They are Joanne O’Meara and Orbax, both physicists at the University of Guelph; Kate Howells of the Planetary Society; and the business executive George Staikos.

The Guelph quartet talk about their outreach activities – including a popular series of activities at local breweries – and about the challenges of raising awareness and funding for their ambitious project.

Also in this week’s episode, Physics World’s Sarah Tesh and Matin Durrani talk about our latest quiz, which looks at the opening lines of famous physics-related popular science books. You can take the quiz at “The first-sentence challenge”.

Device-independent QKD brings unhackable quantum Internet closer

Two independent research groups have demonstrated a protocol for distributing quantum-encrypted keys via a method that is sure to leave would-be network hackers in the dark. The protocol, dubbed device independent quantum key distribution, was first proposed three decades ago but had not been realized experimentally before due to technical limitations, which the researchers have now overcome.

Most people use encryption regularly to ensure that information they transfer via the Internet (such as credit card details) does not fall into the wrong hands. The mathematical foundations of present-day encryption are robust enough that the encrypted “keys” cannot be cracked, even with the fastest supercomputers. This classical encryption may, however, be at risk from future quantum computers.

One solution to this problem is quantum key distribution (QKD), which uses the quantum properties of photons, rather than mathematical algorithms, as the basis for encryption. For example, if a sender uses entangled photons to transmit a key to a receiver, any hacker who tries to spy on this communication will be easy to detect because their intervention will disturb the entanglement. QKD therefore allows the two parties to generate secure, secret keys that they can use to share information.

Vulnerable devices

But there’s a catch. Even if information is sent in a secure way, someone could still gain knowledge of the key by hacking the devices of the sender and/or receiver. Because QKD generally assumes that devices maintain perfect calibration, any deviations can be difficult to detect, leaving them prone to being compromised.

An alternative is device independent QKD (DIQKD), which as its name implies operates independently of the state of the device. DIQKD works as follows. Two users, traditionally named Alice and Bob, each possess one particle of an entangled pair. They measure the particles independently using a strict set of experimental conditions. These measurements are divided into those that are used to generate a key for encryption and those that are used to confirm entanglement. If the particles are entangled, the measured values will violate conditions known as Bell’s inequalities. Establishing this violation guarantees that the key-generation process has not been tampered with.

Schematic diagram showing a photo of John Stewart Bell being encrypted at Alice's end, transmitted securely, and then decrypted at the Bob node to reconstitute the image..
DIQKD schematic: In the experiment, Alice transmits data (such as a photo of the late physicist John Stewart Bell, whose theoretical arguments about the limits to correlations in nature lie at the heart of device-​independent security) to Bob in encrypted form, using the keys generated and transmitted by DIQKD. (Courtesy: University of Oxford; original photo of John Stewart Bell courtesy of CERN)

High-fidelity entanglement, low bit error rate

In the new research, which is described in Nature, an international team from the University of Oxford (UK), CEA (France) and the EPFL, the University of Geneva and ETH (all in Switzerland) performed their measurements on a pair of trapped strontium-88 ions spaced two metres apart. When these ions are excited to a higher electronic state, they spontaneously decay, emitting a photon apiece. A Bell-state measurement (BSM) is then performed on both photons to entangle the ions. To ensure all information is kept within the setup, the ions are then guided to a different location where they are used to perform the DIQKD measurement protocol. After this the sequence is repeated.

Over a period of nearly eight hours, the team created 1.5 million entangled Bell pairs and used them to generate a shared key 95 884 bits long. This was possible because the fidelity of the entanglement was high, at 96%, while the quantum bit error rate was low, at 1.44%. The Bell inequality measurements, meanwhile, produced a value of 2.64, well above the classical limit of 2, meaning the entanglement was not hampered.

In a separate experiment, also described in Nature, researchers at Germany’s Ludwig-Maximilian University (LMU) and the National University of Singapore (NUS) used a pair of optically trapped rubidium-87 atoms located in laboratories 400 metres apart and connected by a 700-metre-long optical fibre. Similar to the other team’s protocol, the atoms are excited and the photons they emit as they decay back to their ground state are used to perform a BSM that entangles the two atoms. The atom’ states are then measured by ionizing them to a particular state. Since ionized atoms are lost from the trap, a fluorescence measurement to check for the presence of the atom completes the protocol.

The LMU-NUS team repeated this sequence 3 342 times over a measurement period of 75 hours, maintaining an entanglement fidelity of 89.2% and a quantum bit error rate of 7.8% throughout. The Bell inequality measurement yielded a result of 2.57, again proving the entanglement remained intact over the measurement period.

Now make it practical

For DIQKD to become a practical encryption method, both teams agree that key generation rates will need to increase. So, too will the distances between Alice and Bob. One way of optimizing the system might be to use cavities to improve photon collection rates. Another step would be to parallelize the entanglement generation process by using arrays of single atoms/ions, rather than pairs. In addition, both teams generate photons at wavelengths with high losses inside optical fibres: 422 nm for strontium and 780 nm for rubidium. This could be addressed through quantum frequency conversion, which shifts photons into the near-infrared region where optical fibres used for telecommunication exhibit much lower loss.

Tim van Leent, a PhD student at LMU and a co-lead author of the LMU-NUS paper, notes that the keys the Oxford-CEA-Switzerland team generated were secure under so-called finite-key security assumptions, which he calls “a great achievement”. He adds that the other team’s work on implementing all necessary steps in the QKD protocol sets an important precedent, pointing out that the entanglement quality reported in this experiment is the highest so far between distant matter-based quantum memories.

Nicolas Sangouard, a physicist at CEA who is one of the lead investigators of the project, says that the LMU-NUS researchers succeeded in showing that entangled states can be distributed over hundreds of metres with a quality that is, in principle, high enough to perform device-independent quantum key distribution. He adds that the difficulties they had to overcome serve as a good illustration of the challenges that device-independent QKD still poses for quantum networking platforms. Extracting a key from the raw data remains particularly difficult, he adds, as the number of experimental repetitions is not enough to extract a key from the measurement results.

Excitonic insulators are created in moiré superlattices

Excitonic insulators – an exotic type of matter with a ground state comprising bound electron–hole pairs – have been made by two independent research groups. The excitonic insulators were created in layered materials called van der Waals heterostructures and the research could potentially lead to the discovery of new quantum phases of matter such as excitonic superfluids. Excitonic insulators could also have practical engineering applications.

Excitons are normally formed in an insulator or semiconductor when an electron is promoted to a higher energy band (by a photon, for example), leaving behind a positively charged “hole”. The electron and hole bind together to create an exciton that behaves much like a particle, which is why excitons are classified as quasiparticles.

Back in the 1960s, the British theoretical physicist and future Nobel laureate Nevill Mott reasoned that, if the band structure of a material were tuned such that, at certain points, the upper energy level is below the lower energy level, then the ground state of the system would contain excitons. Electrons and holes would remain bound together by the Coulomb interaction, but it would be energetically unfavourable for them to recombine. As the excitons would be neutrally charged, they would not carry an electric current and the resulting material would be an insulator.

This remained purely hypothetical until last year, when two groups – one led by Sanfeng Wu and colleagues at Princeton University in New Jersey, the other by David Cobden’s group at the University of Washington in Seattle – independently found evidence suggesting that monolayer tungsten ditelluride showed features consistent with an excitonic insulating state at temperatures below 100 K.

Moiré superlattices

In new research, two independent teams – one led by researchers at University of California, Berkeley; the other a collaboration between scientists in the US, China and Japan – have taken similar, three-layer approaches to creating excitonic insulators. Both groups used heterostructures in which the top two layers formed a moiré bilayer. These bilayers are created when multiple monolayers – here tungsten disulfide and tungsten diselenide – are twisted relative to one another. This creates “superlattices” as the individual lattices move in and out of phase. Such materials have a periodic band structure, which can allow them to form another type of exotic correlated insulator called a Mott insulator in which electrons are confined by this periodicity.

Both groups also used a monolayer of tungsten diselenide as the bottom layer. The Berkeley-led researchers insulated the moiré bilayer from the monolayer using an atomically-thin layer of hexagonal boron nitride. They applied a variable potential across the heterostructure, changing which layer was electron doped and which layer was hole doped. Using optical reflectance spectroscopy, the researchers observed electrons in one layer binding to holes in the other at temperatures below 60 K. “When you dope the moiré bilayer with holes, you get a hole at each lattice site,” explains Berkeley’s Zuocheng Zhang. “If we electron-dope the Mott insulator, the holes will generally be gone. But the holes in the monolayer tend to stay above the part that does not have a hole to minimize the very strong Coulomb interaction.”

The US–China–Japan team performed similar experiments, using an applied electric field to tune the energy levels of the two adjacent layers of tungsten diselenide to create interlayer excitons. They did not use a hexagonal boron nitride layer, however. “These two materials are naturally separated by a van der Waals gap, which is insulating,” says team member Sufei Shi of Rensselaer Polytechnic Institute in New York. “If you have boron nitride in between, that’s going to increase the spatial separation, but it’s going to decrease the Coulomb interaction.” The benefits of this trade-off, the researchers believe, may be evident in their higher transition temperature of 90 K.

Neutral bosons

Both teams now aim to use their platforms to further study the properties of excitonic insulators – which they believe could be markedly different from those of other exotic insulators such as Mott insulators. “In Mott insulators, everything is electrons, so they’re still fermions,” says Yongtao Cui of University of California, Riverside – who is a member of the US–China–Japan team. “In the excitonic insulator, the fundamental units are bound states of electrons and holes, which are charge neutral, and they’re bosons.” This could potentially lead to exotic states of matter such as excitonic superfluids. Fellow team member Chuanwei Zhang of the University of Texas at Dallas says that several obstacles remain, such as increasing the density of the excitations and reducing their temperature, making them delocalized enough to reach quantum degeneracy. The Berkeley researchers also plan to study exciton superfluidity.

“I think these are beautiful, important [studies],” says Wu. “The theoretical idea was half a century ago, and we are still at the stage of trying to conclusively find [excitonic insulators], and with excitonic insulators there are many different types.” He says researchers need to develop more unambiguous detection techniques. Moreover, he believes the current work could find engineering applications in “excitonics”. This aims to create devices that use excitons to transfer information without the heat loss associated with charge transfer. However, practical excitonics has been limited by the short lifetime of optically excited excitons. Wu points out that the latest work offers a way around this problem, “Compared to all known excitons in semiconductors, these excitons in ground states can live for a very long time,” he says. “If you could make them flow, you could make functional devices without charge.”

The research is described in two papers Nature Physics. One paper is by the Berkeley-led team and the other paper is by the US–China–Japan team.

Magnets, magnets, magnets: we’ll need lots of them for a green economy

I was recently in Newcastle to attend PEMD2022 – the 11th international conference on power electronics, machines and drives. What struck me was not only the huge performance improvements that have been happening in electric motors and generators but just how far we still have to go to make transport fully carbon-free.

Global sales of electric cars (including fully battery powered, fuel cell and plug-in hybrids) doubled in 2021 to an all-time high of 6.6 million. They now account for 5–6% of vehicle sales, with more being sold each week than in the whole of 2012, according to the Global Electric Vehicle Outlook 2022 report.

Each new electric vehicle will need at least one high-power electric motor

Projections vary, but annual sales are expected to increase to 65 million electric vehicles by 2030 globally, according to market research firm IHS Markit. Annual sales of vehicles with internal combustion engines, in contrast, will decline from 68 million units in 2021 to 38 million by 2030.

What’s obvious is that each new electric vehicle will need at least one high-power electric motor. Almost all (about 85%) of these vehicles currently use motors with permanent magnet (PMs) as they are the most efficient (the record is 98.8%). A few use Alternating Current (AC) induction motors and generators, but they are 4–8% less efficient than PM motors, up to 60% heavier and up to 70% larger.

Still, these non-PM motors and generators are perfect for, say, trucks, ships and wind-turbine generators. They are also easy to recycle as they can, in principle, be made of one material (say aluminium) and then melted down when they come to the end of their life. Some firms, like Tesla Motors, are even combining the PM and electromagnetic approaches in ever more complex designs to optimize performance and range. None of the advances in electric vehicles would, however, be possible without the huge advances in solid-state power electronics.

Magnetic attraction

Magnets have come a long way since a shepherd in Magnesia in northern Greece noticed the nails in his shoe and the metal tip of his staff were stuck fast to a magnetic rock (or so legend has it). These “lodestones” were used for thousands of years in compasses to navigate but it was not until the early 1800s that Hans Christian Ørsted discovered that an electric current can influence a compass needle.

The first demonstration of a motor with rotary motion occurred in 1821 when Michael Faraday dipped a free-hanging wire into a pool of mercury, on which a PM was placed. The first DC electric motor that could turn machinery was developed by British scientist William Sturgeon in 1832. US inventors Thomas and Emily Davenport built the first practical battery-powered DC electric motor at about the same time.

These motors were used to run machine tools and a printing press. But as the battery power was so expensive, the motors were commercially unsuccessful, and the Davenports ended up bankrupt. Other inventors who tried to develop battery-powered DC motors struggled with the cost of the power source too. Eventually, in the 1880s, attention turned to AC motors, which took advantage of the fact that AC can be sent over long distances at high voltage.

The first AC “induction motor” was invented by the Italian physicist Galileo Ferraris in 1885, with the electric current to drive the motor obtained by electromagnetic induction from the magnetic field of the stator winding. The beauty of this device is that it can be made without any electrical connections to the rotor – a commercial opportunity seized upon by Nikola Tesla. Having independently invented his own induction motor in 1887, he patented the AC motor the following year.

For many years, though, PMs had fields no higher than naturally occurring magnetite (about 0.005 T). It wasn’t until the development of alnico (alloys of mostly aluminium, nickel and cobalt) in the 1930s that practically useful PM DC motors and generators became a possibility. In the 1950s low-cost, ferrite (ceramic) PMs appeared, followed in the 1960s by samarium and cobalt magnets, which were stronger again.

But the real game-changer occurred in the 1980s with the invention of neodymium PMs, which contain neodymium, iron and boron. These days, the N42 grade of neodymium PMs has a strength of some 1.3 T, although that’s not the only key metric when it comes to magnet and motor design: operating temperature is vital too.

Prices of some rare-earth materials have skyrocketed, prompting a huge amount of research into new magnet compositions

That’s because the performance of PMs falls as they warm-up and once they go above “Curie point” (about 320 °C for neodymium magnets), they completely demagnetize – rendering the motor useless. Another important thing about all rare-earth magnets, including neodymium, cobalt and samarium, is that they have a high coercivity, meaning they don’t demagnetize easily when in operation. To make the highest coercivity and best temperature performance magnets you also need small amounts of other heavy rare earths such as dysprosium, terbium and praseodymium.

A question of supply

Trouble is, rare-earth elements are in short supply. It’s not because they are intrinsically rare, their name simply comes from their location in the periodic table. According to a report last year from Magnetics & Materials LLC, by 2030 the world will need 55,000 more tonnes of neodymium magnets than are likely to be available, with 40% of the total demand expected to come from electric vehicles and 11% from wind turbines.

China currently makes 90% of all the world’s neodymium magnets, which is why the US, the EU and others are all trying to develop their capabilities in the supply chain so as not to be disadvantaged. Prices of some rare-earth materials have skyrocketed, prompting a huge amount of research into new magnet compositions, recycling of existing magnets and advanced AC induction motors.

Whichever way you look at it, we’re going to need a lot of magnets if we are to green the economy.

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